Python executable cleanups

[Chroxvi]

This is a follow-up to #30 to fully implement PEP394 in cotainr.

• Use python3 executable instead of python.
• Use sys.executable when applicable
• Update docs to reflect that we implement PEP394
• Added an error early check to produce a meaningful error message when trying to run cotainr using a too old version of Python, i.e. instead of

  Traceback (most recent call last):
  File "./bin/cotainr", line 12, in <module>
    from cotainr.cli import main
  File "<fstring>", line 1
    (sub_cmd=)
            ^
  SyntaxError: invalid syntax

  you now get something like

  Cotainr requires Python>=3.8.0
  You are running Python==3.7.12 | packaged by conda-forge | (default, Oct 26 2021, 06:08:53) 
  [GCC 9.4.0]
  from '/miniconda3/envs/py37_cotainr_test/bin/python3'
  ABORTING!

[Chroxvi]

Add documentation change also to state that we now only support python3

We have always only supported python>=3.8 as stated on https://cotainr.readthedocs.io/en/latest/user_guide/index.html#cotainr-dependencies. The only change is that we now explicitly call python3 instead of relying on python to be a symlink to python3 as python may or may not exist and may or may not point to python2 instead of python3.